Antenna device and method for manufacturing same

ABSTRACT

Provided is an antenna device having a structure that suppresses deterioration in performance as a profile becomes lower. Four FM band antenna elements and two AM band antenna elements are arranged side by side on an antenna base. Further, a circuit board including synthesis circuits in one-to-one correspondence with the antenna elements, respectively, is arranged in an inner space of the antenna elements. In order that omnidirectionality may be exhibited in a horizontal plane, each of the antenna elements includes a helically wound linear conductor, and a planar conductor that is electrically connected to the linear conductor at a part substantially the farthest from a ground plane (antenna base.

TECHNICAL FIELD

The present invention relates to a low profile antenna device mountableon, for example, a vehicle body, and a method of manufacturing theantenna device.

BACKGROUND ART

Antenna devices disclosed in, for example, Patent Literature 1 andPatent Literature 2 are antenna devices for an FM band and for an AMband mountable on a vehicle body. In the antenna device disclosed inPatent Literature 1, an antenna base and an antenna element includingtwo kinds of helical antenna portions are arranged in a shark finantenna case. The antenna element includes a first helical portioncloser to the antenna base and a second helical portion farther from theantenna base. The first helical portion is formed of a rail-like patternor a plate-like conductive member. Meanwhile, the second helical portionhas a surface area per unit length that is larger than that of the firsthelical portion, and is formed of a linear pattern, a solid pattern, asolid pattern and wire, or a plate-like conductive member bent so as tobe substantially U-shaped (oblong helical element).

Further, in the antenna device disclosed in Patent Literature 2, theantenna element includes a helical antenna element and a plate-likeelement. The antenna element is wound about a virtual axis extendingfrom an antenna base toward a top portion of an antenna device for avehicle. The plate-like element is a conductive plate, and is arrangedon an open end side of the helical antenna element so as to cover thetop portion while being electrically connected to the open end side ofthe helical antenna element and so as to have, with the virtual axis, apositional relationship of intersecting each other perpendicularly orobliquely.

CITATION LIST Patent Literature

[PTL 1] JP 2012-161075 A, [PTL 2] JP 2013-106146 A

SUMMARY OF INVENTION Technical Problem

The antenna device disclosed in Patent Literature 1 mainly focuses onfunctioning the entire antenna element as an antenna with efficiency ina limited space. However, in such an antenna device, two kinds ofhelical portions are arranged in a height direction at a predeterminedinterval. In particular, when the second helical portion is formed of aplate-like conductive member, a planar portion thereof is upright withrespect to the antenna base in the so-called vertically orientedstructure. Therefore, there is a limit to the extent of a low profile,and only a height of about 70 mm can be realized.

The antenna device disclosed in Patent Literature 2 has low profile andcan secure a substantially constant antenna gain over a wide band due toan effect of a plate-like element mounted on a tip of the antennaelement. However, this antenna device includes the single antennaelement and the plate-like element, and thus, there is a limit to theextent of a high gain of the antenna. For example, in order to secure anantenna gain equivalent to that of the related-art shark fin antenna, aheight of 50 mm or more is necessary.

In view of the circumstances described above, it is an object of thepresent invention to provide an antenna device having a structure thatcan maintain an antenna gain and other kinds of antenna performanceequivalent to those of the related-art antenna device even when theprofile of the antenna device is lower than the above-mentioned heights.It is another object of the present invention to provide a method ofmanufacturing the above-mentioned antenna device.

Solution to Problem

According to one embodiment of the present invention, there is providedan antenna device, including: an antenna device, comprising: an antennabase having a planar portion at a ground potential in operation; and nantenna elements, where n is a natural number equal to or larger than 2,the antenna elements being arranged on the antenna base so as to exhibitomnidirectionality on a plane in parallel with the planar portion, andbeing configured to receive or transmit the same signal at the sametime, wherein each of the n antenna elements includes: a linearconductor having both end portions arranged in directions away from theplanar portion; and a planar conductor, which is electrically connectedto one end portion of the linear conductor, at a part, at which the oneend portion is substantially the farthest from the planar portion, andwhich is opposed to and substantially in parallel with the planarportion, and wherein another end portion of the linear conductor iselectrically separated from another end portion of a linear conductor ofanother antenna element.

According to one embodiment of the present invention, there is provideda method of manufacturing an antenna device, including the stages of:dividing an area of a planar portion in which antenna elements are to bearranged into k pieces, where k is a natural number equal to or largerthan 2, based on a relationship between a height of the antenna elementsand an antenna gain secured by the area; and arranging side by side, onthe planar portion, k planar conductors each having a divided area and alinear conductor having one end electrically connected to correspondingone of the k planar conductors at a part substantially the farthest fromthe planar portion and another end electrically connected to anamplifier circuit of k lines of amplifier circuits at a partsubstantially the nearest to the planar portion, such thatomnidirectionality is exhibited on a plane in parallel with the planarportion, wherein k antenna elements configured to receive or transmitthe same signal is formed on the planar portion.

Advantageous Effects of Invention

The antenna device according to the present invention includes theplanar conductor opposed to and substantially in parallel with theplanar portion at the ground potential in operation. Since a capacity toground is secured by the planar conductor, the band becomes wider andthe antenna gain is improved. In addition, the plurality of antennaelements each having such a planar conductor are arranged on the antennabase so as to exhibit omnidirectionality within a plane in parallel withthe planar portion, to thereby further improve the antenna gain. Thus,even when the antenna has a small height from the ground plane,reduction of the antenna gain and the like accompanying the small heightof the antenna can be compensated for.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an external perspective view of an antenna device according toa first embodiment of the present invention.

FIG. 2 is an exploded view of the antenna device according to the firstembodiment.

FIG. 3A is a front view of an antenna body portion in the antenna deviceaccording to the first embodiment.

FIG. 3B is a side view of an antenna body portion in the antenna deviceaccording to the first embodiment.

FIG. 3C is a side view of an antenna body portion in the antenna deviceaccording to the first embodiment.

FIG. 4 is a block diagram of an electronic circuit mounted on a circuitboard.

FIG. 5A is a graph for showing a relationship between the top capacityplate area and an antenna gain of each of the first embodiment and anantenna device for comparison, and is for the case of an FM band.

FIG. 5B is a graph for showing a relationship between the top capacityplate area and an antenna gain of each of the first embodiment and anantenna device for comparison, and is for the case of an AM band.

FIG. 6 is a block diagram of an electronic circuit mounted on a circuitboard of the related-art antenna device.

FIG. 7A is a graph for showing directional characteristics of theantenna device according to the first embodiment, and is for the case ofthe FM band.

FIG. 7B is a graph for showing directional characteristics of theantenna device according to the first embodiment, and is for the case ofthe AM band.

FIG. 8A is a front view of an antenna body portion in an antenna deviceaccording to a second embodiment of the present invention.

FIG. 8B is a side view of an antenna body portion in an antenna deviceaccording to a second embodiment of the present invention.

FIG. 8C is a side view of an antenna body portion in an antenna deviceaccording to a second embodiment of the present invention.

FIG. 9 is an external perspective view of an antenna device according toa third embodiment of the present invention.

FIG. 10 is an exploded view of the antenna device according to the thirdembodiment.

FIG. 11A is a front view of an antenna body portion in the antennadevice according to the third embodiment.

FIG. 11B is a side view of an antenna body portion in the antenna deviceaccording to the third embodiment.

FIG. 11C is a side view of an antenna body portion in the antenna deviceaccording to the third embodiment.

FIG. 12 is a block diagram of an electronic circuit mounted on a circuitboard.

FIG. 13 is a graph for showing a relationship of antenna gains betweenthe third embodiment and a reference antenna.

FIG. 14 is a block diagram of a high-frequency circuit.

FIG. 15 is a block diagram of another electronic circuit mounted on thecircuit board.

FIG. 16 is a block diagram of another electronic circuit mounted on thecircuit board.

DESCRIPTION OF EMBODIMENTS

Now, embodiments of the present invention are described with referenceto the drawings.

First Embodiment

In a first embodiment of the present invention, a case is described inwhich the present invention is applied to a low profile antenna devicethat can be used in an FM band (from 76 MHz to 108 MHz) and in an AMband (0.520 MHz to 1.710 MHz) as an example. The antenna device ismounted, for example, on a vehicle roof and used thereon, and exhibitsomnidirectionality in a horizontal plane.

FIG. 1 is an external perspective view for illustrating the exemplarystructure of an antenna device according to this embodiment, and FIG. 2is an exploded perspective view thereof. An antenna device 1 includes anantenna base 10 formed of a metal member, such as aluminum die cast. Theantenna base 10 is a component for mounting the antenna device 1 on avehicle roof, and has an upper surface (direction opposite to a bottomsurface toward the vehicle roof, the same applies to the following),which has formed thereon a planar portion that is electrically connectedto the vehicle roof at a ground potential in operation (“antennamounting planar portion”) and a cover joining portion for watertightlyjoining a cover portion 50 thereto.

The antenna base 10 is in the shape of a square with four sides eachhaving a length of 160 mm. The antenna mounting planar portion is formedso as to have the area of 22,500 mm² (=150 mm×150 mm) in a region thatis slightly recessed from the cover joining portion on the outerperiphery of the antenna base 10.

The antenna mounting planar portion has a thickness of about 0.5 mm, andthe cover joining portion has a thickness of about 1.0 mm.

A mounting hole 10 a for inserting a mounting mechanism (not shown inthe drawings) for mounting the antenna device 1 on a vehicle roof isformed substantially in the center of the antenna mounting planarportion. A circuit board 20 is arranged on the antenna mounting planarportion. The circuit board 20 has a thickness of about 0.5 mm.

The circuit board 20 is one resin substrate having an electronic circuitmounted thereon. Specifically, a substrate surface is partitioned intosix, each of which has formed therein an amplifier circuit including anantenna feeding terminal and an amplifier electrically connected via awiring pattern. In other words, six lines of amplifier circuits areformed. Further, on the circuit board 20, also formed thereon onesynthesis circuits configured to synthesize output signals (amplifiedsignals) of the respective lines and an output terminal for transmittingan output of the synthesis circuit to an external device.

Six element supports 301 to 306 (referred to as element supports 30 whenthere is no need to distinct the element supports) are arranged side byside on an upper surface of the circuit board 20. “Arranged side byside” means being arranged in the same plane without an overlap.

The element supports 301 to 306 are formed of dielectric blocks or thelike, and support corresponding antenna elements 401 to 406 (referred toas antenna elements 40 when there is no need to distinct the antennaelements), respectively.

Each of the element supports 30 has, when formed of a dielectric block,a top portion opposed to and in parallel with the antenna base 10 and aframe extending downward (direction toward the circuit board 20, thesame applies to the following) from the outer periphery of the topportion. The top portion may be a surface having an opening, and theframe may be a combination of a plurality of columns.

A portion surrounded by the frame of each of the element supports 30 isa hollow space. Circuit components protruding from the circuit board 20are housed in the hollow space. This can reduce the size of the entireantenna device. A helical groove is formed in an outer surface of theframe at a predetermined pitch.

Each of the antenna elements 40 includes a linear conductor, and aplanar conductor for securing a capacity to ground. The planar conductoris, for example, a mesh-like or plate-like conductor in the shape of arectangular flat plate having the area that is approximately equal tothat of the top portion of the element support 30 (area of the topportion surrounded by the outer periphery thereof) and a thickness of0.2 mm (hereinafter referred to as “top capacity plate”).

According to this embodiment, four FM band antenna elements 401 to 404and two AM band antenna elements 405 and 406 are arranged side by side.Each of the top capacity plates of the FM band antenna elements 401 to404 has the area of 3,500 mm² (=70 mm×50 mm). Each of the top capacityplates of the AM band antenna elements 405 and 406 has the area of 2,800mm² (=70 mm×40 mm).

A gap of from 5 mm to 10 mm is provided between the adjacent antennaelements 40. In other words, as a whole, the six antenna elements 40 arearranged side by side in an antenna mounting planar portion of 22,500mm². The reason of the area is described later.

One end portion of the linear conductor having both end portions iselectrically connected to the top capacity plate at a part substantiallythe farthest from the antenna base 10. “A part substantially thefarthest” means a part at a height at which the highest earth capacitycan be secured. The other end portion of the linear conductor isconnected to the antenna feeding terminal (not shown in the drawings)formed in the circuit board 20.

The linear conductor for the FM band is, for example, a copper wirehaving a diameter of 0.4 mm wound several times in the groove on theouter periphery of the frame of each of the element supports 30, thatis, a helical coil wound at predetermined intervals (pitch). The copperwire is fitted into the groove in the frame, and thus, by forming thegroove to have a depth corresponding to the diameter of the copper wire,a helical diameter of the copper wire becomes approximately the same asan outer diameter of the top capacity plate. The helical diameter andthe pitch are adjusted so as to produce resonance at a frequency in theFM band.

When the linear conductor is a helical coil, it is desired that windingdirections of adjacent copper wires (helical coils) be opposite to eachother. This causes currents flowing through the copper wires to be inphase with each other. Thus, compared with a case in which currents indifferent phases flow through the copper wires, coupling between theantenna elements is suppressed to suppress deterioration in antennaperformance.

It is enough that a linear conductor for the AM band can secure aspecific inductance component, and thus, the linear conductor is notnecessarily required to be a helical coil. However, in the case of ahelical coil, it is desired that winding directions be opposite to eachother.

FIGS. 3A-3C are illustrations of an outer appearance of a portion of theassembled antenna device from which the cover portion 50 is removed.FIG. 3A is a top view, and FIG. 3B and FIG. 3C are side views thereof.As illustrated in FIG. 3A, each of the top capacity plates of theantenna elements 40 is a substantially rectangular flat plate, and isformed into the same shape and size as those of the top portion of theelement support 30 protruding from the antenna base 10. Therefore, thetop capacity plate is opposed to and in parallel with the antennamounting planar portion of the antenna base 10. Further, the topcapacity plate secures the capacity to ground in operation.

The top capacity plate is a rectangular flat plate in the illustratedcase. However, from the viewpoint of securing necessary electricalperformance, the top capacity plate is not necessarily required to be arectangular flat plate, and may be a flat plate in the shape of acircle, a polygon, a ring, a mesh, a combination of a ring and alattice, or other shapes. In this case, the shape of the top portion ofthe element support 30 conforms to the shape of the top capacity plate.

The cover portion 50 covers the antenna base 10, the circuit board 20,and the antenna elements 40, and is watertightly joined to the coverjoining portion formed on the outer periphery of the antenna base 10.The cover portion 50 is formed of, for example, a radio wavetransmitting synthetic resin, and is formed into the shape of a box. Thecolor of the cover portion 50 may match the color of the vehicle body(in FIG. 1, for the sake of convenience of description, the coverportion 50 is formed of a translucent resin). Further, the cover portionmay have not a single structure but a double structure.

FIG. 4 is an illustration of the exemplary structure of the electroniccircuit mounted on the circuit board 20. Signals received by the FM bandantenna elements 401 to 404 are input to FM amplifiers 201 to 204 inone-to-one correspondence with the antenna elements 401 to 404,respectively, to be amplified. Output from the FM amplifier 201 andoutput from the FM amplifier 202 are synthesized by a synthesis circuit211. Output from the FM amplifier 203 and output from the FM amplifier204 are synthesized by a synthesis circuit 212. Output from thesynthesis circuit 211 and output from the synthesis circuit 212 arefurther synthesized by a synthesis circuit 221.

Signals received by the AM band antenna elements 405 and 406 are alsoinput to AM amplifiers 205 and 206 in a pair with the antenna elements405 and 406, respectively, to be amplified. Output from the AM amplifier205 and output from the AM amplifier 206 are synthesized by a synthesiscircuit 213. Output from the synthesis circuit 221 and output from thesynthesis circuit 213 are output to an output terminal 231.

A band-pass filter, an automatic gain controller (AGC), or the like areadded to the electronic circuit as appropriate.

Here, the FM amplifiers 201 to 204 are described in detail. It ispreferred that an amplifying element in the first stage of each of theFM amplifiers 201 to 204 be an element that attains low noise in a widefrequency range of, for example, from 76 MHz to 108 MHz. Specifically,an element that attains a minimum noise figure Fmin of 0.2 dB or lessand an equivalent noise resistance Rn of 4Ω or less in the receivedfrequency band is preferred. Such elements include, for example, a highelectron mobility transistor (HEMT) manufactured of a GaAs-based,InP-based, GaN-based, or Si—Ge-based compound semiconductor. An HEMT isa field effect transistor (FET) using high mobility two-dimensionalelectron gas induced by semiconductor heterojunction as a channel, andis an element that is generally used in a high frequency band above theFM band.

An HEMT is used in this embodiment because in operation, a high priorityis given to achieve a substantially constant noise figure (hereinafterreferred to as “NF”) in an entire desired frequency band when connectedto an antenna element, rather than pursuit of input/output impedancematching. NF is an index expressed as a ratio between a signal-to-noiseratio (Si/Ni) in input to the amplifier and a signal-to-noise ratio(So/No) in output from the amplifier. As NF becomes smaller, lower noisecharacteristics are attained.

As the AM amplifiers 205 and 206, taking into consideration 1/f noise,that is, noise that attenuates at 3 dB/octave being noise in a lowfrequency band, not HEMTs but ordinary FETs or bipolar transistors areused.

Next, antenna performance of the antenna device 1 according to thisembodiment is described in detail.

It is well known that, as a distance to an antenna element farthest fromthe plane at the ground potential, that is, the height of the antennaelement becomes smaller, the matching frequency range between anelectronic circuit connected to the antenna element and the antennaelement becomes smaller. It is also well known that antenna gain isproportional to the second power of the height.

In the antenna device 1 according to this embodiment, the matchingfrequency range that becomes smaller as the profile of the antennaelement becomes lower is enlarged through an increase of the area of thetop capacity plate to secure the capacity to ground. The antenna gainthat is reduced as the profile becomes lower is compensated for byreceiving the same signal by a plurality of antenna elements (with theincreased areas of the top capacity plates as described above) andsynthesizing the received signals after being amplified. Theamplification may be performed after the synthesis. With this, practicalantenna performance can be obtained even when the profile is low. Thereason is described below.

FIGS. 5A and 5B are characteristics diagrams for showing therelationship between the area of the top capacity plate and the antennagain when the height (H) of the antenna elements 40 having the structuredescribed above is changed to 10 mm, 20 mm, and 30 mm. “Height” as usedherein means a distance from the antenna mounting planar portion at theground potential to the top capacity plate. FIG. 5A is for the case ofthe FM band, and FIG. 5B is for the case of the AM band. In each of thefigures, the horizontal axis denotes the top capacity plate area in mm²and the vertical axis denotes the antenna gain in dB. The antenna gaindB is an average gain in the band.

The characteristics diagrams are calculated using “HFSS”, which is athree-dimensional electromagnetic field simulator manufactured by ANSYS,Inc. For comparison purposes, an antenna gain of an antenna device bothfor the FM band and for the AM band having a height from the plane atthe ground potential (corresponding to the antenna mounting planarportion) of 60 mm and the top capacity plate area of 10×40 (=400) mm² isused as a reference (0 dB). Such an antenna device is referred to as a“reference antenna” for the sake of convenience.

When, while the top capacity plate area is kept 400 mm², the profilebecomes lower from 60 mm to 30 mm, 20 mm, and 10 mm, the antenna gainbecomes lower accordingly. For example, in the cases shown in FIG. 5Aand FIG. 5B, when the height is as small as 10 mm, the antenna gainbecomes approximately 1/36, that is, −15 dB both for the FM band and forthe AM band.

On the other hand, with regard to all the heights, as the top capacityplate area becomes larger, the matching frequency range becomes widerand the antenna gain can be improved.

However, although, as the top capacity plate area becomes larger, theantenna gain is improved more for the AM band, the extent of improvementin antenna gain is decreased for the FM band when the top capacity platearea is approximately 3,500 mm². This means that, when a space forhousing the antenna element is limited as in an antenna device mountedon a vehicle, a sufficient antenna gain cannot be secured simply byincreasing the area more than necessary.

According to this embodiment, the antenna elements are not sharedbetween the FM band and the AM band, and the FM band antenna elementsand the AM band antenna elements are independent of each other. In orderto secure the top capacity plate area of 3,500 mm² for each of the FMband antenna elements 401 to 404, the top capacity plates have a longside of 70 mm and a short side of 50 mm. Through use of the top capacityplates of the four antenna elements 401 to 404 described above, theantenna gain can be improved by 6 dB. Specifically, even when the heightfrom the antenna mounting planar portion is as small as 10 mm, thedifference in antenna gain with the reference antenna having a height of60 mm can be reduced to about −9 dB. However, even when the area isabout 90% of the above (3,000 mm²), the difference in antenna gaindescribed above is about −9.5 dB, and top capacity plates having such asize can also be used.

In the case of the AM band, through use of top capacity plates having along side of 70 mm and a short side of 40 mm (=2,800 mm²), even when theheight from the antenna mounting planar portion is as small as 10 mm,the difference in antenna gain with the reference antenna having aheight of 60 mm can be reduced to −3 dB. However, even when the area isabout 90% of the above (2,500 mm²), the difference in antenna gaindescribed above is about −4 dB, and top capacity plates having such asize can also be used.

Further, through obtainment of an output signal by arranging the antennaelements 401 to 404 with the top capacity plates having the areasdescribed above in one-to-one correspondence with the amplifiers 201 to204 and synthesizing the amplified signals of the amplifiers 201 to 204,the antenna gain can be improved by four times (6 dB) for the FM bandand by two times (3 dB) for the AM band.

With this, the antenna gain in the FM band can be improved from −9 dB to−3 dB. The antenna gain can be enhanced compared with a case in whichone antenna element having the same area is used. Specifically, the areawhen the four antenna elements 401 to 404 each having a top capacityplate of 70 mm×50 mm are arranged side by side is 14,000 mm². When oneantenna element forms a top capacity plate having such an area, as isclear from the graph of FIG. 5A for the case when the height is 10 mm,the antenna gain is −7.5 dB. Therefore, even when the area is the same,when the four antennas are used, the gain becomes higher by 4.5 dB.

As can be seen from FIG. 5A and FIG. 5B, when, in the limited area ofthe antenna mounting planar portion, the individual top capacity plateareas are attempted to be reduced from the size described above toincrease the number of the antenna elements, and the number ofamplifiers corresponding thereto is attempted to be increased, theantenna gain may become lower as the top capacity plate becomes smaller,and loss of the synthesis circuits becomes larger, and thus, sufficientantenna performance cannot be obtained as a whole. Therefore, there is alimit to the number of the antenna elements.

Meanwhile, when the antenna elements have a height of 20 mm or 30 mm,the number of antenna elements and amplifiers corresponding thereto canbe reduced.

Next, a mechanism of improving the antenna performance on the electroniccircuit side is described.

The related-art low profile antenna device for the FM band and the AMband mounted on a vehicle typically has the structure illustrated inFIG. 6 because a space for housing the antenna element and the like islimited (the same can be said with regard to the reference antennadescribed above). Specifically, in the related-art antenna device, oneantenna element 601 is used both for the FM band and for the AM band.After received signals are separated into FM band signals and AM bandsignals by a duplexer circuit 602, the FM band signals are input to anFM amplifier 603 and the AM band signals are input to an AM amplifier604. An output from the FM amplifier 603 and an output from the AMamplifier 604 are guided to an external electronic device via an outputterminal 605.

However, the duplexer circuit 602 is a combination of a high-pass filterand a low-pass filter using a lumped constant, and thus, it is generallydifficult to completely separate the FM band signals and the AM bandsignals. As a result, part of the FM band signals flow into the AMamplifier. Similarly, part of the AM band signals flow into the FMamplifier. Therefore, energy of the received signals is partly lost. Asa result, energy of signals at the output terminal 605 is not a sum ofthe output from the FM amplifier 603 and the output from the AMamplifier 604.

Meanwhile, in the antenna device 1 according to this embodiment, the FMband antenna elements 401 to 404 and the AM band antenna elements 405and 406 are used. FM band signals are independently amplified by the FMamplifiers 201 to 204, whereas AM band signals are independentlyamplified by the AM amplifiers 205 and 206, and after that, synthesis isperformed on the resultants by the respective synthesis circuits 211 to213 and 221. Therefore, a signal-to-noise ratio (S/N) is improved, whichleads to improvement in antenna gain.

This is described taking a pair of antenna elements as an example. TheS/N of one antenna element and one amplifier is expressed by thefollowing expression.

So/No=GSi/(GNi+Na)   (1)

In Expression 1, So represents an output signal, No represents outputnoise, Si represents an input signal, Ni represents input noise, Narepresents amplifier noise, and G represents an amplification gain.

The output signal So is simply the input signal Si multiplied by G,while the output noise No is the input noise Ni multiplied by G with thenoise Na caused by the amplifier added thereto. In this case, when twopairs of the antenna element and the amplifier are connected inparallel, the input signal Ni and the input noise No are both additionof the same values, and are thus simply the sum total. However, thenoises Na caused by the amplifiers are random and are not related toeach other. Therefore, the result is not simply the sum total, and isthe square root of the sum of the mean squares, i.e., √2Na.

Specifically, the S/N when two pairs of the antenna element and theamplifier are connected in parallel is expressed by the followingexpression.

So/No=2GSi/(2GNi+√2Na)   (2)

When Expression (1) and Expression (2) are compared with each other, itcan be seen that the output S/N is larger in Expression (2) (parallelconnection).

According to actual measurements by the inventors of the presentinvention, it was found that, by omitting the duplexer circuit 602illustrated in FIG. 6 and connecting in parallel the FM band antennaelements 401 to 404 and the amplifiers 201 to 204, and the AM bandantenna elements 405 and 406 and the amplifiers 205 and 206, the antennagain was able to be improved by 3 dB each.

FIG. 7A and FIG. 7B are graphs for showing directional characteristicsin the horizontal plane for the case of the FM band and for the case ofthe AM band, respectively. With the antenna elements 40 having thestructure illustrated in FIG. 1 to FIG. 3C, substantially the samereceiving sensitivity can be obtained omnidirectionally both for the FMband and for the AM band. In other words, the antenna device 1 accordingto this embodiment is omnidirectional in a plane in parallel with theantenna mounting planar portion.

Therefore, electromagnetic waves can be omnidirectionally receivedwithout arranging, for example, directional antenna elements in aplurality of directions.

As described above, the antenna device 1 according to this embodimentwas able to improve the antenna gain by about 6 dB by setting the topcapacity plate area of each of the FM band antenna elements 401 to 404to 3,150 mm² or more, preferably 3,500 mm² or more, by 6 dB by arrangingthe four FM band antenna elements 401 to 404 in the same plane, andfurther, by 3 dB by omitting the duplexer circuit. In other words, itwas found that the antenna performance equivalent to that of thereference antenna having a height of 60 mm was able to be maintainedeven when the profile was as low as 10 mm.

Further, the antenna device 1 according to this embodiment was able toimprove the antenna gain by about 12 dB by setting the top capacityplate area of each of the AM band antenna elements 405 and 406 to 2,520mm² or more, preferably 2,800 mm² or more, by 3 dB by arranging the twoAM band antenna elements 405 and 406 in the same plane, and by 3 dB byomitting the duplexer circuit. In other words, it was found that theantenna performance equivalent to or more than that of the referenceantenna having a height of 60 mm was able to be maintained even when theprofile was as low as 10 mm.

Second Embodiment

Next, an embodiment of the present invention is described in which thebasic structure as an antenna device for the FM band and for the AM bandis the same as that of the first embodiment and the height of theantenna elements, that is, the distance from the antenna mounting planarportion to the top capacity plate is larger than that of the antennadevice 1 according to the first embodiment. Names of structural elementsof the antenna device and the like are similar to those in the firstembodiment.

As can be seen from the characteristics diagrams of FIG. 5A and FIG. 5B,when the antenna element has a height that is larger than 10 mm, the topcapacity plate area for compensation can be reduced, that is, the areaof the antenna mounting planar portion can be reduced. Accordingly, in asecond embodiment of the present invention, a case is described in whichthe antenna element has a height of 20 mm and the antenna mountingplanar portion has the area of 10,000 mm² (=100 mm×100 mm) as anexample.

FIGS. 8A-8C are illustrations of an outer appearance of a portion of anantenna device according to the second embodiment from which a coverportion is removed. FIG. 8A is a top view, and FIG. 8B and FIG. 8C areside views thereof.

As illustrated in FIG. 8A, in the antenna device according to the secondembodiment, one AM band antenna element 403 a is arranged between two FMband antenna elements 401 a and 402 a so as to be side by side on anantenna mounting planar portion of an antenna base 210. An FM amplifierthat is the same as that described in the first embodiment is connectedto each of the FM band antenna elements 401 a and 402 a. Outputs fromthese FM amplifiers are synthesized by a synthesis circuit. Further, anAM amplifier that is the same as that described in the first embodimentis connected to the AM band antenna element 403 a.

The FM band antenna elements 401 a and 402 a are formed by arranging topcapacity plates at top portions of element supports 301 a and 302 aformed of dielectric blocks and winding linear conductors (helicalcoils) around frames of the element supports 301 a and 302 a,respectively. Further, the AM band antenna element 403 a is formed so asto include a top capacity plate arranged at a top portion of an elementsupport 303 a and a linear conductor (helical coil) having one endelectrically connected to the top capacity plate and another endelectrically connected to a circuit board via a hollow portion in theelement support 303 a.

Each of the top capacity plates of the FM band antenna elements 401 aand 402 a has a long side of 100 mm and a short side of 27 mm. Further,the top capacity plate of the AM band antenna element 403 a has a longside of 100 mm and a short side of 42 mm.

As illustrated in FIG. 5A, when each of the FM band antenna elements 401a and 402 a has a height of 20 mm and the top capacity plate area of2,700 mm², an antenna gain of one of the antenna elements is −4.5 dB.Therefore, compensation is made by 3 dB through use of two antennaelements having the top capacity plate area, and further, by 3 dBthrough not use of the duplexer circuit. Thus, compensation is made by 6dB in total, and the antenna performance is higher than that of thereference antenna.

It can be seen that, also according to this embodiment, through use of aplurality of antenna elements, the antenna performance can be enhancedcompared with a case in which one antenna element is used. Specifically,in an FM antenna of an antenna device 2 according to this embodiment,the area when the two antenna elements 401 a and 402 a each having thetop capacity plate of 100 mm×27 mm are arranged side by side is 5,400mm². As is clear from FIG. 5A, compared with an antenna gain of −3.5 dBin the case of one antenna element having a height of 20 mm with a topcapacity plate having the area of 5,400 mm², the gain becomes higher by2 dB when the total area is the same but the two antenna elements areused.

Also with regard to the antenna element 403 a for the AM band, when theheight is 20 mm and the top capacity plate area is 4,200 mm², theantenna gain exceeds +4 dB, and thus, the antenna performance can behigher than that of the reference antenna.

Further, while the antenna mounting planar portion has the area of22,500 mm² (=150 mm×150 mm) in the antenna device 1 according to thefirst embodiment, in the antenna device according to the secondembodiment, only the area of 10,000 mm² (=100 mm×100 mm) is necessary.Thus, in exchange for the increase in height by 10 mm, the installationspace of the antenna elements can be reduced by more than a half. Theantenna device according to the second embodiment is alsoomnidirectional in the horizontal plane.

When an antenna device having the same antenna performance is realizedwith the height of the antenna element being 30 mm, the installationspace of the antenna elements can be further reduced.

Specifically, with reference to FIG. 5A and FIG. 5B, for example, forthe FM band, by setting the top capacity plate area to 700 mm², theantenna gain becomes −4 dB. Therefore, through use of two antennaelements each having a top capacity plate of this size, the antenna gainbecomes −1 dB. Through omission of the duplexer circuit, an antenna gainof 3 dB is further obtained, and thus, while securing antennaperformance that is equivalent to that of the reference antenna, theinstallation space of the antenna elements can be further reduced.

Third Embodiment

Next, a third embodiment of the present invention is described. In thisembodiment, there is described a case of an antenna device that can maketransmission/reception in a 800 MHz band, that is, in a frequency bandof from 800 MHz to 1,000 MHz in a cellular system. Names of structuralelements of the antenna device and the like are similar to those in thefirst embodiment. The antenna device according to this embodiment isalso mounted on a conductive antenna mounting plane such as a vehicleroof, and used thereon.

FIG. 9 is an external perspective view for illustrating the exemplarystructure of the antenna device according to the third embodiment, andFIG. 10 is an exploded perspective view thereof. An antenna device 101includes an antenna base 110, a circuit board 120, four element supports1301 to 1304 (referred to as element supports 130 when there is no needto distinct the element supports), four antenna elements 1401 to 1404(referred to as antenna elements 140 when there is no need to distinctthe antenna elements), and a cover portion 150. The cover portion 150 isformed of a radio wave transmitting synthetic resin.

An upper surface of the antenna base 110 has formed thereon an antennamounting planar portion that is electrically connected to the vehicleroof to be at the ground potential in operation and a cover joiningportion for watertightly joining the cover portion 150. The antennamounting planar portion is formed so as to have the area of 900 mm² (=30mm×30 mm) in a region that is slightly recessed from the cover joiningportion on the outer periphery of the antenna base 110. The antennamounting planar portion has a thickness of about 0.5 mm, and the coverjoining portion has a thickness of about 1.0 mm.

A mounting hole 110 a for inserting a mounting mechanism (not shown inthe drawings) for mounting the antenna device 101 on a vehicle roof isformed substantially in the center of the antenna mounting planarportion. The circuit board 120 is arranged on the antenna mountingplanar portion. The circuit board 120 has a thickness of about 0.5 mm.

Similarly to the cases of the first and second embodiments, each of theantenna elements 140 includes a top capacity plate and a linearconductor. The top capacity plate is formed of, for example, a copperplate having a thickness of 0.2 mm with four sides each having a lengthof 13 mm (having the area of 13×13 mm²). The linear conductor is formedof, for example, a copper wire having a diameter of 0.1 mm and is woundseveral times around each of the element supports 130. One end of thelinear conductor is connected to a top capacity plate in a pair, andanother end thereof is connected to an antenna feeding terminal formedon the circuit board 120. Winding directions of adjacent linearconductors are opposite to each other. In this manner, currents flowingthrough the copper wires are in phase with each other. Thus, comparedwith a case in which currents in different phases flow through thecopper wires, coupling between antenna elements is suppressed tosuppress deterioration in antenna performance.

Each of the element supports 130 has the function of a positioning guidewhen the corresponding linear conductor is wound therearound, and thefunction of holding and fixing the corresponding top capacity plate, andis formed of a hollow dielectric block protruding in a directionperpendicular to the antenna mounting plane or the like. A height fromthe antenna mounting plane to the top capacity plate is about 10 mm.

The circuit board 120 is a substrate having mounted thereontransmission/reception terminals connected to the antenna elements 140,an electronic circuit including distribution/synthesis circuitsconfigured to distribute a signal in transmission and configured tosynthesize a signal in reception, and an output terminal for passing asignal to/from an external circuit. The circuit board 120 is housed inhollow portions in the element supports 130. With this, the entire sizeof the antenna device can be cut down.

FIGS. 11A-11C are illustrations of an outer appearance of an assembledantenna body. FIG. 11A is a top view, and FIG. 11B and FIG. 11C are sideviews thereof. As illustrated in FIG. 11A, each of the top capacityplates is a substantially rectangular flat plate, and is formed into thesame shape and size as those of the top portion of the element support130 protruding from the antenna base 110. Therefore, the top capacityplate is substantially in parallel with the antenna mounting planarportion.

Similarly to the cases of the first embodiment and the secondembodiment, the top capacity plate is not necessarily required to be arectangular flat plate, and may be in the shape of a circle, a polygon,a ring, a mesh, a combination of a ring and a lattice, or other shapes.

Further, the linear conductor is a helical coil wound around outer sidesurfaces of the element support 130 at predetermined intervals (pitch),and a helical diameter thereof is approximately the same as an outerdiameter of the top capacity plate. In other words, the size of thehelical diameter is equivalent to the top capacity plate area (area ofthe portion surrounded by the outer periphery thereof). The helicaldiameter and the pitch are adjusted so that resonance is produced at afrequency in a cellular band in the case of an antenna element for the800 MHz band.

Next, the structures of the respective portions of the antenna device101 having the structure illustrated in FIG. 9 to FIG. 11C are describedin detail. The top capacity plate and the linear conductor are arrangedas described above, and as a result, the antenna elements 140 is sizedto be 13×13×10 mm³. Space between the antenna elements 140 is 4 mm.Therefore, the antenna mounting planar portion on the antenna base 110has the area of 900 mm² (=30×30 mm²). Further, a housing space of theentire antenna elements 140 is sized to be 30×30×10 mm³.

FIG. 12 is an illustration of the exemplary structure of the electroniccircuit mounted on the circuit board 120. The antenna element 1401 andthe antenna element 1402 are connected to a distribution/synthesiscircuit 1201, and the antenna element 1403 and the antenna element 1404are connected to a distribution/synthesis circuit 1202. Further, the twodistribution/synthesis circuits 1201 and 1202 are connected to adistribution/synthesis circuit 1203. The distribution/synthesis circuit1203 is connected to an external device including a receiver and atransmitter via an output terminal 1204.

When the antenna elements 1401 to 1404 receive signals, thedistribution/synthesis circuits 1201, 1202, and 1203 synthesize thesereceived signals and guide the synthesized signal to the receiver of theexternal device. The same signal is received at the same time, and thus,the antenna gain is greatly enhanced. On the other hand, when a signalis transmitted, a signal to be transmitted that is output from thetransmitter of the external device is distributed to be fed to therespective antenna elements 1401 to 1404. Also in this case, the samesignal is transmitted at the same time, and thus, the antenna gain isgreatly enhanced.

FIG. 13 is a graph for showing the relationship between the antenna gainin the 800 MHz band and the top capacity plate area. The vertical axisdenotes the antenna gain dB compared with that of a reference antenna,and the horizontal axis denotes the area mm². The antenna gain dBrepresents an average gain in the band.

In this embodiment, the reference antenna is one helical antenna that iswound around a square having sides being 13 mm and a height of 10 mm.That is, the helical antenna is the same as the antenna element 140 fromwhich the top capacity plate is removed.

The area of an opening for one reference antenna is 169 mm² (=13 mm×13mm), and thus, a gain A1 thereof is used as a reference 0 dB. In FIG.13, A2 denotes an antenna gain when four reference antennas to each ofwhich the top capacity plate is added are arranged as illustrated inFIG. 9 to FIGS. 11A-11C, and the value thereof is 5.4 dB. A3 denoteschange in antenna gain when the top capacity plate area varies with theheight being maintained at 10 mm.

With reference to FIG. 13, the antenna gain of one antenna element inwhich the top capacity plate is added to the reference antenna is higherby about 1.8 dB. Meanwhile, it is enough that a top capacity plate of anantenna having an antenna gain equivalent to that of the referenceantenna has the area of 80 mm². In other words, through addition of thetop capacity plate, the antenna gain becomes higher and a wider band isattained.

Meanwhile, when the four antenna elements 1401 to 1404 each having thetop capacity plate of 13 mm×13 mm are arranged side by side as in theantenna device 101 according to this embodiment, the area is about 900mm². As is clear from A3 in FIG. 13, an antenna gain of one antennaelement having a top capacity plate of 900 mm² is 4.0 dB. Thus, evenwith the same area, the antenna gain becomes higher by 1.4 dB when thearea is divided into four to be used.

As described above, a wider band is attained also in the antenna device101 according to the third embodiment by increasing the top capacityplate area of the antenna element, and further, dividing the same areainto a plurality of pieces to be used, the antenna gain can be enhanced.

In this embodiment, a case in which the receiver of the external deviceamplifies in reception and the transmitter of the external deviceamplifies in transmission is described as an example, but theseamplifiers may be arranged on the antenna device side. However, in thiscase, it is desired to take measures with regard to shielding of radiowaves in transmission.

FIG. 14 to FIG. 16 are illustrations of the exemplary structure when theamplifiers are arranged on the antenna device side. When theamplification is made on the antenna device side, a high-frequencycircuit having the structure illustrated in FIG. 14 is arranged. Thehigh-frequency circuit is a circuit in which a receiving amplifier R10and a transmitting amplifier T10 are arranged in parallel between a pairof distribution/synthesis circuits RT10 and RT11 connected to terminalsC1 and C2, respectively.

FIG. 15 is an illustration of an example case in which high-frequencycircuits 1211 to 1214 having the structure illustrated in FIG. 14 arearranged immediately under the four antenna elements 1401 to 1404,respectively. A distribution/synthesis circuit 1215 is connected to thehigh-frequency circuit 1211 and the high-frequency circuit 1212, and adistribution/synthesis circuit 1216 is connected to the high-frequencycircuit 1213 and the high-frequency circuit 1214. Further, the twodistribution/synthesis circuits 1215 and 1216 are connected to adistribution/synthesis circuit 1217, and this distribution/synthesiscircuit 1217 is connected to the output terminal 1204 illustrated inFIG. 12.

FIG. 16 is an illustration of an example case in which thehigh-frequency circuits 1221 to 1224 having the structure illustrated inFIG. 14 are arranged immediately under the four antenna elements 1401 to1404, respectively, and are connected to one distribution/synthesiscircuit 1225. The distribution/synthesis circuit 1225 is connected tothe output terminal 1204.

In the cases of FIG. 14 to FIG. 16, the distribution/synthesis circuitsRT10, RT11, 1215 to 1217, and 1225 function as distribution circuits intransmission and function as synthesis circuit in reception.

MODIFIED EXAMPLES

Three embodiments are described above, but the antenna device accordingto the present invention can be modified as in the following.

(1) In the first embodiment, a case is described in which the four FMband antenna elements and two AM band antenna elements are arranged sideby side, and in the second embodiment, a case is described in which thetwo FM band antenna elements and the one AM band antenna element arearranged side by side as examples, but the numbers of the antennaelements may be different from the above. Further, only FM band antennaelements may be arranged on the antenna mounting planar portion to formthe antenna device.

(2) In the first embodiment and the second embodiment, cases aredescribed in which the amplifiers and the synthesis circuits arearranged on the circuit board 20 and 20 a as examples. However, thecircuit board 20 or the electronic circuit mounted thereon may bearranged not on the antenna base 10, 210 but on a portion other than theantenna device to be electrically connected via an interface. Further,only a synthesis circuit configured to synthesize signals in therespective frequency bands may be arranged on the circuit board 20, anda synthesized signal of received signals may be amplified by an externaldevice of the antenna device.

(3) In the first embodiment and the second embodiment, cases of theantenna devices for the AM band and for the FM band are described, andin the third embodiment, a case of the antenna device for the cellular800 MHz band is described as examples. However, the antenna device mayinclude an antenna element that can receive a signal in the GPSfrequency band, a frequency band for a navigation system, or a frequencyband for satellite broadcasting.

Fourth Embodiment

Next, a method of manufacturing the antenna devices described in thefirst to third embodiments is described. These antenna devices can bemanufactured through the following manufacturing steps. For the sake ofconvenience, description is made with regard to the antenna device 1according to the first embodiment, but the same can be said with regardto the antenna devices according to the second embodiment and the thirdembodiment.

(1) Dividing Step

On the antenna base 10, the area of the antenna mounting planar portionin which the antenna elements can be arranged is determined. Then, thearea is divided into k pieces (k is a natural number equal to or largerthan 2) for the respective frequency bands, taking account of spacebetween the elements. Specifically, taking into consideration the mutualrelationship among the antenna gain, the height of the antenna element,and the top capacity plate area shown in FIG. 5A and FIG. 5B, and a gain(3 dB) that can be compensated for in the electronic circuit, the numberof divisions (k) and the top capacity plate area after the division aredetermined from the area of the antenna mounting planar portion that canbe secured.

(2) Arranging Step

After the circuit board 20 having the electronic circuit for the dividedlines mounted thereon is housed on the antenna mounting planar portion,the k top capacity plates each having the divided area and the linearconductors are joined so that omnidirectionality is exhibited on a planein parallel with the antenna mounting planar portion. In other words,the top capacity plates are joined to the element supports 30 so as tobe in parallel with or substantially in parallel with the antennamounting planar portion. One end of the linear conductor is electricallyconnected to the top capacity plate at a part substantially the farthestfrom the antenna mounting planar portion, and another end thereof isconnected to the electronic circuit independently of another ends ofother linear conductors.

In this manner, k antenna elements that can receive the same signal inthe same frequency band at the same time are formed on the antenna base10.

(3) Assembling Step

Finally, the cover portion is joined to the cover joining portion of theantenna base 10 to complete the antenna device 1.

REFERENCE SIGNS LIST

1, 101 . . . antenna device

10, 110 . . . antenna base

20, 120 . . . circuit board

201 to 204 . . . FM amplifier

205, 206 . . . AM amplifier

211 to 213 . . . synthesis circuit

30, 130 . . . element support

40, 140 . . . antenna element

401 to 404, 401 a, 402 a . . . FM band antenna element

405, 406, 403 a . . . AM band antenna element

1401 to 1404 . . . antenna element for cellular communication

50, 150 . . . cover portion

1. An antenna device, comprising: an antenna base having a planarportion at a ground potential in operation; and n antenna elements,where n is a natural number equal to or larger than 2, the antennaelements being arranged on the antenna base so as to exhibitomnidirectionality on a plane in parallel with the planar portion, andbeing configured to receive or transmit the same signal at the sametime, wherein each of the n antenna elements includes: a linearconductor having both end portions arranged in directions away from theplanar portion; and a planar conductor, which is electrically connectedto one end portion of the linear conductor, at a part, at which the oneend portion is substantially the farthest from the planar portion, andwhich is opposed to and substantially in parallel with the planarportion, and wherein another end portion of the linear conductor iselectrically separated from another end portion of a linear conductor ofanother antenna element.
 2. The antenna device according to claim 1,wherein the another end portion of the linear conductor is connected toan input portion of an electronic circuit configured to synthesize asignal input to the electronic circuit and a signal received by theanother antenna element.
 3. The antenna device according to claim 1,wherein the another end portion of the linear conductor is connected toan output portion of an electronic circuit, which is configured todistribute and output a signal to be transmitted, which is transmittedalso from the another antenna element.
 4. The antenna device accordingto claim 2 or 3, wherein each of the n antenna elements is supported bya hollow frame configured to support the linear conductor, wherein thelinear conductor is helically wound along side surfaces of the frame,wherein the planar conductor is joined to a part of the frame that issubstantially the farthest from the planar portion, and wherein theelectronic circuit is housed in a space surrounded by the frame.
 5. Theantenna device according to claim 4, wherein the linear conductorincludes adjacent linear conductors helically wound in directionsopposite to each other.
 6. The antenna device according to claim 1,wherein the signal to be received or transmitted includes a signal in an800 MHz band, and wherein the planar conductor of the antenna elementhas an area of 80 mm² or more when a height of the planar conductor fromthe planar portion is 10 mm.
 7. An antenna device, comprising: anantenna base having a planar portion at a ground potential in operation;and n first antenna elements, where n is a natural number equal to orlarger than 2, and m second antenna element, where m is a natural numberequal to or larger than 1, the first antenna elements being configuredto receive the same signal in a first frequency band at the same time,the second antenna element being configured to receive a signal in asecond frequency band different from the first frequency band, the nfirst antenna elements and the m second antenna element being arrangedon the antenna base so as to exhibit omnidirectionality on a plane inparallel with the planar portion, wherein each of the n first antennaelements and the m second antenna element includes: a linear conductorhaving both end portions arranged in directions away from the planarportion; and a planar conductor, which is electrically connected to oneend portion of the linear conductor, at a part, at which the one endportion is substantially the farthest from the planar portion, and whichis opposed to and substantially in parallel with the planar portion, andwherein another end portion of the linear conductor of each of the nfirst antenna elements is electrically separated from another endportion of the linear conductor of the m second antenna element.
 8. Theantenna device according to claim 7, wherein the linear conductorincudes adjacent linear conductors helically wound in directionsopposite to each other.
 9. The antenna device according to claim 7,wherein the first frequency band includes an FM band and the secondfrequency band includes an AM band, and wherein, when a height of theplanar conductor from the planar portion is 10 mm, each of the planarconductors of the n first antenna elements has an area of 3,000 mm² ormore, and the planar conductor of the m second antenna element has anarea of 2,500 mm² or more.
 10. The antenna device according to claim 7,wherein the another end portion of the linear conductor of each of the nfirst antenna elements is connected to an input portion of an electroniccircuit configured to synthesize a signal input to the electroniccircuit and a signal received by another first antenna element.
 11. Theantenna device according to claim 10, wherein the electronic circuitfurther includes an amplifier having, as an amplifying element in afirst stage, a semiconductor element configured to attain a minimumnoise figure of 0.2 dB or less and an equivalent noise resistance of 4Ωor less in a frequency band to be received.
 12. A method ofmanufacturing an antenna device, comprising: dividing an area of aplanar portion in which antenna elements are to be arranged into kpieces, where k is a natural number equal to or larger than 2, based ona relationship between a height of the antenna elements and an antennagain secured by the area; and arranging side by side, on the planarportion, k planar conductors each having a divided area and a linearconductor having one end electrically connected to corresponding one ofthe k planar conductors at a part substantially the farthest from theplanar portion and another end electrically connected to an amplifiercircuit of k lines of amplifier circuits at a part substantially thenearest to the planar portion, such that omnidirectionality is exhibitedon a plane in parallel with the planar portion, wherein k antennaelements configured to receive or transmit the same signal is formed onthe planar portion.